The invention relates to a local area network operating in the asynchronous transfer mode (ATM) for transmitting cells, comprising a plurality of network interfaces.
A Local Area Network (LAN) operating in the asynchronous transfer mode is known from EP-0 641 105 A2. This network comprises a plurality of stations and assigned network interfaces which comprise each a send and receive ring connection. Via the ring structured in this fashion, cells are transmitted which contain address information about the point of destination, for example, a specific station. A network interface comprises a switch and a controller. The switch comprises a path memory assigned to the receive ring connection and a path memory assigned to a receive station connection. When a cell is received, the respective switch or coupling operations are carried out in the switch based on the path memories. For example, an arriving cell is conveyed to the controller, to the station and/or to the ring on the basis of the address information.
When an asynchronous transfer mode is used in a system, payload information, for example, television, picture or sound signals, are sent in fixed-length blocks along digital signal processing arrangements. A cell containing a given number of bytes (for example, 53 bytes) is denoted a fixed-length block. Each cell comprises a header field with a length of, for example, 5 bytes and an information field in which the payload information is accommodated having a length of, for example, 48 bytes. In such a header field are contained address information, data for error detection and data for control information. Address information comprises trunk identifiers and link identifiers. The link identifier, also referenced VCI (Virtual Channel Identifier) contains the indication about the destination of the cell in the system. For transmitting a cell, a virtual channel is rendered available based on the virtual channel identifier. As a rule, a VCI is changed each time a switching point is arrived at. A group of a plurality of virtual channels is denoted a virtual path. A virtual path is featured by the trunk identifier. Such a virtual path is referenced a VPI (Virtual Path Identifier). Cells are assigned to successively determined time slots. The duration of such a time slot depends on the clock frequency used as a basis for the switching components. If no payload information is available, empty cells are transmitted in such a time slot i.e. cells without payload information. Cells containing payload information are referenced payload cells.
The local area network known from EP 0 641 105 A2 comprises only a single ring system and does not provide any possibility of exchanging cells via a plurality of ring systems. A ring system may then comprise one ring or two rings for cell streams of the opposite direction. A ring comprises a closed signal path via a plurality of network interfaces.
From DE 40 02 022 A1 is known a method and arrangement for the highly available transmission of data between two bus systems operating according to the token passing principle. The two bus systems are connected via two coupling paths provided for exchanging data. One switching path is active and the other is in the stand-by mode. The switching path in the stand-by mode checks the regular transfer of tokens of the active switching path. When the active switching path fails, the switching path operating in the stand-by mode thus far is activated. In this way, always only a single switching path is active in this arrangement.
Therefore, it is an object of the invention to provide a local area network operating in the asynchronous transfer mode, in which also an exchange of cells between link systems is possible.
The object is achieved by a local area network of the type defined in the opening paragraph working in the asynchronous transfer mode, in that at least two ring systems comprising each two network interfaces are provided, which ring systems comprise each at least one bridge connection; and in that a first network interface of a first ring system and a first network interface of a second ring system are coupled via at least a first bridge connection for transmitting the cell stream between the two ring systems, and a second network interface of the first ring system and a second network interface of the second ring system are coupled via at least a second bridge connection for conveying the cell stream between the two ring systems.
The local area network according to the invention comprises at least two ring systems with one or two rings. Cells can be transmitted via respective first network interfaces from one ring system to another ring system. The two first network interfaces are then coupled via at least one bridge connection. The first and second ring systems contain each also second network interfaces which are coupled via at least a second bridge connection. The cell stream may thus flow either via the first or the second network interface. If one bridge connection fails, the cell stream can flow through the other bridge connection when the measures according to the invention are implemented.
If there is an interruption of the first bridge connections, the first network interfaces are not provided to convey cells whose destination is the other ring system to that other ring system. Similarly, if there is an interruption of the second bridge connections, the second network interfaces are not provided to convey cells whose destination is the other ring system to that other ring system. The network interface which is assigned to an uninterrupted bridge connection thus no longer allows any cell to flow to the bridge connection, but conveys a cell onto the ring.
In a further embodiment of the invention there is provided that a first or second network interface comprises a switch for switching at least one receive and one send ring connection and two bridge connections, in that a receive ring connection and a bridge connection are assigned each a path memory for controlling the paths in the switch after address information has been taken from the header field of a cell, and in that the path memories of the first and second network interfaces of a ring system have like contents. Since the path memories in the network interfaces of a ring system have the same contents, the cell stream can be conveyed without delay via a second bridge connection if a first bridge connection fails.
The invention also relates to a method of transmitting cells in a local area network operating in the asynchronous transfer mode (ATM), which network comprises a plurality of network interfaces. For this purpose, cells are transmitted between a first network interface of a first ring system and a first network interface of a second ring system via at least a first bridge connection, and between a second network interface of the first ring system and a second network interface of the second ring system via at least a second bridge connection.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.